llvm.org GIT mirror llvm / 66c5fd6 lib / Debugger / UnixLocalInferiorProcess.cpp
66c5fd6

Tree @66c5fd6 (Download .tar.gz)

UnixLocalInferiorProcess.cpp @66c5fd6raw · history · blame

  1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
//===-- UnixLocalInferiorProcess.cpp - A Local process on a Unixy system --===//
//
//                     The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file provides one implementation of the InferiorProcess class, which is
// designed to be used on unixy systems (those that support pipe, fork, exec,
// and signals).
//
// When the process is started, the debugger creates a pair of pipes, forks, and
// makes the child start executing the program.  The child executes the process
// with an IntrinsicLowering instance that turns debugger intrinsics into actual
// callbacks.
//
// This target takes advantage of the fact that the Module* addresses in the
// parent and the Module* addresses in the child will be the same, due to the
// use of fork().  As such, global addresses looked up in the child can be sent
// over the pipe to the debugger.
//
//===----------------------------------------------------------------------===//

#include "llvm/Debugger/InferiorProcess.h"
#include "llvm/Constant.h"
#include "llvm/Instructions.h"
#include "llvm/Module.h"
#include "llvm/ModuleProvider.h"
#include "llvm/Type.h"
#include "llvm/CodeGen/IntrinsicLowering.h"
#include "llvm/ExecutionEngine/GenericValue.h"
#include "llvm/ExecutionEngine/ExecutionEngine.h"
#include "llvm/Support/FileUtilities.h"
#include "llvm/ADT/StringExtras.h"
#include "FDHandle.h"
#include <cerrno>
#include <csignal>
#include <unistd.h>        // Unix-specific debugger support
#include <sys/types.h>
#include <sys/wait.h>
using namespace llvm;

// runChild - Entry point for the child process.
static void runChild(Module *M, const std::vector<std::string> &Arguments,
                     const char * const *envp,
                     FDHandle ReadFD, FDHandle WriteFD);

//===----------------------------------------------------------------------===//
//                        Parent/Child Pipe Protocol
//===----------------------------------------------------------------------===//
//
// The parent/child communication protocol is designed to have the child process
// responding to requests that the debugger makes.  Whenever the child process
// has stopped (due to a break point, single stepping, etc), the child process
// enters a message processing loop, where it reads and responds to commands
// until the parent decides that it wants to continue execution in some way.
//
// Whenever the child process stops, it notifies the debugger by sending a
// character over the wire.
//

namespace {
  /// LocationToken - Objects of this type are sent across the pipe from the
  /// child to the parent to indicate where various stack frames are located.
  struct LocationToken {
    unsigned Line, Col;
    const GlobalVariable *File;
    LocationToken(unsigned L = 0, unsigned C = 0, const GlobalVariable *F = 0)
      : Line(L), Col(C), File(F) {}
  };
}

// Once the debugger process has received the LocationToken, it can make
// requests of the child by sending one of the following enum values followed by
// any data required by that command.  The child responds with data appropriate
// to the command.
//
namespace {
  /// CommandID - This enum defines all of the commands that the child process
  /// can respond to.  The actual expected data and responses are defined as the
  /// enum values are defined.
  ///
  enum CommandID {
    //===------------------------------------------------------------------===//
    // Execution commands - These are sent to the child to from the debugger to
    // get it to do certain things.
    //

    // StepProgram: void->char - This command causes the program to continue
    // execution, but stop as soon as it reaches another stoppoint.
    StepProgram,

    // FinishProgram: FrameDesc*->char - This command causes the program to
    // continue execution until the specified function frame returns.
    FinishProgram,

    // ContProgram: void->char - This command causes the program to continue
    // execution, stopping at some point in the future.
    ContProgram,

    // GetSubprogramDescriptor: FrameDesc*->GlobalValue* - This command returns
    // the GlobalValue* descriptor object for the specified stack frame.
    GetSubprogramDescriptor,

    // GetParentFrame: FrameDesc*->FrameDesc* - This command returns the frame
    // descriptor for the parent stack frame to the specified one, or null if
    // there is none.
    GetParentFrame,

    // GetFrameLocation - FrameDesc*->LocationToken - This command returns the
    // location that a particular stack frame is stopped at.
    GetFrameLocation,

    // AddBreakpoint - LocationToken->unsigned - This command instructs the
    // target to install a breakpoint at the specified location.
    AddBreakpoint,

    // RemoveBreakpoint - unsigned->void - This command instructs the target to
    // remove a breakpoint.
    RemoveBreakpoint,
  };
}




//===----------------------------------------------------------------------===//
//                            Parent Process Code
//===----------------------------------------------------------------------===//

namespace {
  class IP : public InferiorProcess {
    // ReadFD, WriteFD - The file descriptors to read/write to the inferior
    // process.
    FDHandle ReadFD, WriteFD;

    // ChildPID - The unix PID of the child process we forked.
    mutable pid_t ChildPID;
  public:
    IP(Module *M, const std::vector<std::string> &Arguments,
       const char * const *envp);
    ~IP();

    std::string getStatus() const;

    /// Execution method implementations...
    virtual void stepProgram();
    virtual void finishProgram(void *Frame);
    virtual void contProgram();


    // Stack frame method implementations...
    virtual void *getPreviousFrame(void *Frame) const;
    virtual const GlobalVariable *getSubprogramDesc(void *Frame) const;
    virtual void getFrameLocation(void *Frame, unsigned &LineNo,
                                  unsigned &ColNo,
                                  const GlobalVariable *&SourceDesc) const;

    // Breakpoint implementation methods
    virtual unsigned addBreakpoint(unsigned LineNo, unsigned ColNo,
                                   const GlobalVariable *SourceDesc);
    virtual void removeBreakpoint(unsigned ID);


  private:
    /// startChild - This starts up the child process, and initializes the
    /// ChildPID member.
    ///
    void startChild(Module *M, const std::vector<std::string> &Arguments,
                    const char * const *envp);

    /// killChild - Kill or reap the child process.  This throws the
    /// InferiorProcessDead exception an exit code if the process had already
    /// died, otherwise it just kills it and returns.
    void killChild() const;

  private:
    // Methods for communicating with the child process.  If the child exits or
    // dies while attempting to communicate with it, ChildPID is set to zero and
    // an exception is thrown.

    /// readFromChild - Low-level primitive to read some data from the child,
    /// throwing an exception if it dies.
    void readFromChild(void *Buffer, unsigned Size) const;

    /// writeToChild - Low-level primitive to send some data to the child
    /// process, throwing an exception if the child died.
    void writeToChild(void *Buffer, unsigned Size) const;

    /// sendCommand - Send a command token and the request data to the child.
    ///
    void sendCommand(CommandID Command, void *Data, unsigned Size) const;

    /// waitForStop - This method waits for the child process to reach a stop
    /// point.
    void waitForStop();
  };
}

// create - This is the factory method for the InferiorProcess class.  Since
// there is currently only one subclass of InferiorProcess, we just define it
// here.
InferiorProcess *
InferiorProcess::create(Module *M, const std::vector<std::string> &Arguments,
                        const char * const *envp) {
  return new IP(M, Arguments, envp);
}

/// IP constructor - Create some pipes, them fork a child process.  The child
/// process should start execution of the debugged program, but stop at the
/// first available opportunity.
IP::IP(Module *M, const std::vector<std::string> &Arguments,
       const char * const *envp)
  : InferiorProcess(M) {

  // Start the child running...
  startChild(M, Arguments, envp);

  // Okay, we created the program and it is off and running.  Wait for it to
  // stop now.
  try {
    waitForStop();
  } catch (InferiorProcessDead &IPD) {
    throw "Error waiting for the child process to stop.  "
          "It exited with status " + itostr(IPD.getExitCode());
  }
}

IP::~IP() {
  // If the child is still running, kill it.
  if (!ChildPID) return;

  killChild();
}

/// getStatus - Return information about the unix process being debugged.
///
std::string IP::getStatus() const {
  if (ChildPID == 0)
    return "Unix target.  ERROR: child process appears to be dead!\n";

  return "Unix target: PID #" + utostr((unsigned)ChildPID) + "\n";
}


/// startChild - This starts up the child process, and initializes the
/// ChildPID member.
///
void IP::startChild(Module *M, const std::vector<std::string> &Arguments,
                    const char * const *envp) {
  // Create the pipes.  Make sure to immediately assign the returned file
  // descriptors to FDHandle's so they get destroyed if an exception is thrown.
  int FDs[2];
  if (pipe(FDs)) throw "Error creating a pipe!";
  FDHandle ChildReadFD(FDs[0]);
  WriteFD = FDs[1];

  if (pipe(FDs)) throw "Error creating a pipe!";
  ReadFD = FDs[0];
  FDHandle ChildWriteFD(FDs[1]);

  // Fork off the child process.
  switch (ChildPID = fork()) {
  case -1: throw "Error forking child process!";
  case 0:  // child
    delete this;       // Free parent pipe file descriptors
    runChild(M, Arguments, envp, ChildReadFD, ChildWriteFD);
    exit(1);
  default: break;
  }
}

/// sendCommand - Send a command token and the request data to the child.
///
void IP::sendCommand(CommandID Command, void *Data, unsigned Size) const {
  writeToChild(&Command, sizeof(Command));
  writeToChild(Data, Size);
}

/// stepProgram - Implement the 'step' command, continuing execution until
/// the next possible stop point.
void IP::stepProgram() {
  sendCommand(StepProgram, 0, 0);
  waitForStop();
}

/// finishProgram - Implement the 'finish' command, executing the program until
/// the current function returns to its caller.
void IP::finishProgram(void *Frame) {
  sendCommand(FinishProgram, &Frame, sizeof(Frame));
  waitForStop();
}

/// contProgram - Implement the 'cont' command, continuing execution until
/// a breakpoint is encountered.
void IP::contProgram() {
  sendCommand(ContProgram, 0, 0);
  waitForStop();
}


//===----------------------------------------------------------------------===//
// Stack manipulation methods
//

/// getPreviousFrame - Given the descriptor for the current stack frame,
/// return the descriptor for the caller frame.  This returns null when it
/// runs out of frames.
void *IP::getPreviousFrame(void *Frame) const {
  sendCommand(GetParentFrame, &Frame, sizeof(Frame));
  readFromChild(&Frame, sizeof(Frame));
  return Frame;
}

/// getSubprogramDesc - Return the subprogram descriptor for the current
/// stack frame.
const GlobalVariable *IP::getSubprogramDesc(void *Frame) const {
  sendCommand(GetSubprogramDescriptor, &Frame, sizeof(Frame));
  const GlobalVariable *Desc;
  readFromChild(&Desc, sizeof(Desc));
  return Desc;
}

/// getFrameLocation - This method returns the source location where each stack
/// frame is stopped.
void IP::getFrameLocation(void *Frame, unsigned &LineNo, unsigned &ColNo,
                          const GlobalVariable *&SourceDesc) const {
  sendCommand(GetFrameLocation, &Frame, sizeof(Frame));
  LocationToken Loc;
  readFromChild(&Loc, sizeof(Loc));
  LineNo     = Loc.Line;
  ColNo      = Loc.Col;
  SourceDesc = Loc.File;
}


//===----------------------------------------------------------------------===//
// Breakpoint manipulation methods
//
unsigned IP::addBreakpoint(unsigned LineNo, unsigned ColNo,
                           const GlobalVariable *SourceDesc) {
  LocationToken Loc;
  Loc.Line = LineNo;
  Loc.Col = ColNo;
  Loc.File = SourceDesc;
  sendCommand(AddBreakpoint, &Loc, sizeof(Loc));
  unsigned ID;
  readFromChild(&ID, sizeof(ID));
  return ID;
}

void IP::removeBreakpoint(unsigned ID) {
  sendCommand(RemoveBreakpoint, &ID, sizeof(ID));
}


//===----------------------------------------------------------------------===//
//             Methods for communication with the child process
//
// Methods for communicating with the child process.  If the child exits or dies
// while attempting to communicate with it, ChildPID is set to zero and an
// exception is thrown.
//

/// readFromChild - Low-level primitive to read some data from the child,
/// throwing an exception if it dies.
void IP::readFromChild(void *Buffer, unsigned Size) const {
  assert(ChildPID &&
         "Child process died and still attempting to communicate with it!");
  while (Size) {
    ssize_t Amount = read(ReadFD, Buffer, Size);
    if (Amount == 0) {
      // If we cannot communicate with the process, kill it.
      killChild();
      // If killChild succeeded, then the process must have closed the pipe FD
      // or something, because the child existed, but we cannot communicate with
      // it.
      throw InferiorProcessDead(-1);
    } else if (Amount == -1) {
      if (errno != EINTR) {
        ChildPID = 0;
        killChild();
        throw "Error reading from child process!";
      }
    } else {
      // We read a chunk.
      Buffer = (char*)Buffer + Amount;
      Size -= Amount;
    }
  }
}

/// writeToChild - Low-level primitive to send some data to the child
/// process, throwing an exception if the child died.
void IP::writeToChild(void *Buffer, unsigned Size) const {
  while (Size) {
    ssize_t Amount = write(WriteFD, Buffer, Size);
    if (Amount < 0 && errno == EINTR) continue;
    if (Amount <= 0) {
      // If we cannot communicate with the process, kill it.
      killChild();

      // If killChild succeeded, then the process must have closed the pipe FD
      // or something, because the child existed, but we cannot communicate with
      // it.
      throw InferiorProcessDead(-1);
    } else {
      // We wrote a chunk.
      Buffer = (char*)Buffer + Amount;
      Size -= Amount;
    }
  }
}

/// killChild - Kill or reap the child process.  This throws the
/// InferiorProcessDead exception an exit code if the process had already
/// died, otherwise it just returns the exit code if it had to be killed.
void IP::killChild() const {
  assert(ChildPID != 0 && "Child has already been reaped!");

  // If the process terminated on its own accord, closing the pipe file
  // descriptors, we will get here.  Check to see if the process has already
  // died in this manner, gracefully.
  int Status = 0;
  int PID;
  do {
    PID = waitpid(ChildPID, &Status, WNOHANG);
  } while (PID < 0 && errno == EINTR);
  if (PID < 0) throw "Error waiting for child to exit!";

  // Ok, there is a slight race condition here.  It's possible that we will find
  // out that the file descriptor closed before waitpid will indicate that the
  // process gracefully died.  If we don't know that the process gracefully
  // died, wait a bit and try again.  This is pretty nasty.
  if (PID == 0) {
    usleep(10000);   // Wait a bit.

    // Try again.
    Status = 0;
    do {
      PID = waitpid(ChildPID, &Status, WNOHANG);
    } while (PID < 0 && errno == EINTR);
    if (PID < 0) throw "Error waiting for child to exit!";
  }

  // If the child process was already dead, then indicate that the process
  // terminated on its own.
  if (PID) {
    assert(PID == ChildPID && "Didn't reap child?");
    ChildPID = 0;            // Child has been reaped
    if (WIFEXITED(Status))
      throw InferiorProcessDead(WEXITSTATUS(Status));
    else if (WIFSIGNALED(Status))
      throw InferiorProcessDead(WTERMSIG(Status));
    throw InferiorProcessDead(-1);
  }

  // Otherwise, the child exists and has not yet been killed.
  if (kill(ChildPID, SIGKILL) < 0)
    throw "Error killing child process!";

  do {
    PID = waitpid(ChildPID, 0, 0);
  } while (PID < 0 && errno == EINTR);
  if (PID <= 0) throw "Error waiting for child to exit!";

  assert(PID == ChildPID && "Didn't reap child?");
}


/// waitForStop - This method waits for the child process to reach a stop
/// point.  When it does, it fills in the CurLocation member and returns.
void IP::waitForStop() {
  char Dummy;
  readFromChild(&Dummy, sizeof(char));
}


//===----------------------------------------------------------------------===//
//                             Child Process Code
//===----------------------------------------------------------------------===//

namespace {
  class SourceSubprogram;

  /// SourceRegion - Instances of this class represent the regions that are
  /// active in the program.
  class SourceRegion {
    /// Parent - A pointer to the region that encloses the current one.
    SourceRegion *Parent;

    /// CurSubprogram - The subprogram that contains this region.  This allows
    /// efficient stack traversals.
    SourceSubprogram *CurSubprogram;

    /// CurLine, CurCol, CurFile - The last location visited by this region.
    /// This is used for getting the source location of callers in stack frames.
    unsigned CurLine, CurCol;
    void *CurFileDesc;

    //std::vector<void*> ActiveObjects;
  public:
    SourceRegion(SourceRegion *p, SourceSubprogram *Subprogram = 0)
     : Parent(p), CurSubprogram(Subprogram ? Subprogram : p->getSubprogram()) {
      CurLine = 0; CurCol = 0;
      CurFileDesc = 0;
    }

    virtual ~SourceRegion() {}

    SourceRegion *getParent() const { return Parent; }
    SourceSubprogram *getSubprogram() const { return CurSubprogram; }

    void updateLocation(unsigned Line, unsigned Col, void *File) {
      CurLine = Line;
      CurCol = Col;
      CurFileDesc = File;
    }

    /// Return a LocationToken for the place that this stack frame stopped or
    /// called a sub-function.
    LocationToken getLocation(ExecutionEngine *EE) {
      LocationToken LT;
      LT.Line = CurLine;
      LT.Col = CurCol;
      const GlobalValue *GV = EE->getGlobalValueAtAddress(CurFileDesc);
      LT.File = dyn_cast_or_null<GlobalVariable>(GV);
      return LT;
    }
  };

  /// SourceSubprogram - This is a stack-frame that represents a source program.
  ///
  class SourceSubprogram : public SourceRegion {
    /// Desc - A pointer to the descriptor for the subprogram that this frame
    /// represents.
    void *Desc;
  public:
    SourceSubprogram(SourceRegion *P, void *desc)
      : SourceRegion(P, this), Desc(desc) {}
    void *getDescriptor() const { return Desc; }
  };


  /// Child class - This class contains all of the information and methods used
  /// by the child side of the debugger.  The single instance of this object is
  /// pointed to by the "TheChild" global variable.
  class Child {
    /// M - The module for the program currently being debugged.
    ///
    Module *M;

    /// EE - The execution engine that we are using to run the program.
    ///
    ExecutionEngine *EE;

    /// ReadFD, WriteFD - The file descriptor handles for this side of the
    /// debugger pipe.
    FDHandle ReadFD, WriteFD;

    /// RegionStack - A linked list of all of the regions dynamically active.
    ///
    SourceRegion *RegionStack;

    /// StopAtNextOpportunity - If this flag is set, the child process will stop
    /// and report to the debugger at the next possible chance it gets.
    volatile bool StopAtNextOpportunity;

    /// StopWhenSubprogramReturns - If this is non-null, the debugger requests
    /// that the program stops when the specified function frame is destroyed.
    SourceSubprogram *StopWhenSubprogramReturns;

    /// Breakpoints - This contains a list of active breakpoints and their IDs.
    ///
    std::vector<std::pair<unsigned, LocationToken> > Breakpoints;

    /// CurBreakpoint - The last assigned breakpoint.
    ///
    unsigned CurBreakpoint;

  public:
    Child(Module *m, ExecutionEngine *ee, FDHandle &Read, FDHandle &Write)
      : M(m), EE(ee), ReadFD(Read), WriteFD(Write),
        RegionStack(0), CurBreakpoint(0) {
      StopAtNextOpportunity = true;
      StopWhenSubprogramReturns = 0;
    }

    /// writeToParent - Send the specified buffer of data to the debugger
    /// process.
    ///
    void writeToParent(const void *Buffer, unsigned Size);

    /// readFromParent - Read the specified number of bytes from the parent.
    ///
    void readFromParent(void *Buffer, unsigned Size);

    /// childStopped - This method is called whenever the child has stopped
    /// execution due to a breakpoint, step command, interruption, or whatever.
    /// This stops the process, responds to any requests from the debugger, and
    /// when commanded to, can continue execution by returning.
    ///
    void childStopped();

    /// startSubprogram - This method creates a new region for the subroutine
    /// with the specified descriptor.
    ///
    void startSubprogram(void *FuncDesc);

    /// startRegion - This method initiates the creation of an anonymous region.
    ///
    void startRegion();

    /// endRegion - This method terminates the last active region.
    ///
    void endRegion();

    /// reachedLine - This method is automatically called by the program every
    /// time it executes an llvm.dbg.stoppoint intrinsic.  If the debugger wants
    /// us to stop here, we do so, otherwise we continue execution.
    ///
    void reachedLine(unsigned Line, unsigned Col, void *SourceDesc);
  };

  /// TheChild - The single instance of the Child class, which only gets created
  /// in the child process.
  Child *TheChild = 0;
} // end anonymous namespace


// writeToParent - Send the specified buffer of data to the debugger process.
void Child::writeToParent(const void *Buffer, unsigned Size) {
  while (Size) {
    ssize_t Amount = write(WriteFD, Buffer, Size);
    if (Amount < 0 && errno == EINTR) continue;
    if (Amount <= 0) {
      write(2, "ERROR: Connection to debugger lost!\n", 36);
      abort();
    } else {
      // We wrote a chunk.
      Buffer = (const char*)Buffer + Amount;
      Size -= Amount;
    }
  }
}

// readFromParent - Read the specified number of bytes from the parent.
void Child::readFromParent(void *Buffer, unsigned Size) {
  while (Size) {
    ssize_t Amount = read(ReadFD, Buffer, Size);
    if (Amount < 0 && errno == EINTR) continue;
    if (Amount <= 0) {
      write(2, "ERROR: Connection to debugger lost!\n", 36);
      abort();
    } else {
      // We read a chunk.
      Buffer = (char*)Buffer + Amount;
      Size -= Amount;
    }
  }
}

/// childStopped - This method is called whenever the child has stopped
/// execution due to a breakpoint, step command, interruption, or whatever.
/// This stops the process, responds to any requests from the debugger, and when
/// commanded to, can continue execution by returning.
///
void Child::childStopped() {
  // Since we stopped, notify the parent that we did so.
  char Token = 0;
  writeToParent(&Token, sizeof(char));

  StopAtNextOpportunity = false;
  StopWhenSubprogramReturns = 0;

  // Now that the debugger knows that we stopped, read commands from it and
  // respond to them appropriately.
  CommandID Command;
  while (1) {
    SourceRegion *Frame;
    const void *Result;
    readFromParent(&Command, sizeof(CommandID));

    switch (Command) {
    case StepProgram:
      // To step the program, just return.
      StopAtNextOpportunity = true;
      return;

    case FinishProgram:         // Run until exit from the specified function...
      readFromParent(&Frame, sizeof(Frame));
      // The user wants us to stop when the specified FUNCTION exits, not when
      // the specified REGION exits.
      StopWhenSubprogramReturns = Frame->getSubprogram();
      return;

    case ContProgram:
      // To continue, just return back to execution.
      return;

    case GetSubprogramDescriptor:
      readFromParent(&Frame, sizeof(Frame));
      Result =
        EE->getGlobalValueAtAddress(Frame->getSubprogram()->getDescriptor());
      writeToParent(&Result, sizeof(Result));
      break;

    case GetParentFrame:
      readFromParent(&Frame, sizeof(Frame));
      Result = Frame ? Frame->getSubprogram()->getParent() : RegionStack;
      writeToParent(&Result, sizeof(Result));
      break;

    case GetFrameLocation: {
      readFromParent(&Frame, sizeof(Frame));
      LocationToken LT = Frame->getLocation(EE);
      writeToParent(&LT, sizeof(LT));
      break;
    }
    case AddBreakpoint: {
      LocationToken Loc;
      readFromParent(&Loc, sizeof(Loc));
      // Convert the GlobalVariable pointer to the address it was emitted to.
      Loc.File = (GlobalVariable*)EE->getPointerToGlobal(Loc.File);
      unsigned ID = CurBreakpoint++;
      Breakpoints.push_back(std::make_pair(ID, Loc));
      writeToParent(&ID, sizeof(ID));
      break;
    }
    case RemoveBreakpoint: {
      unsigned ID = 0;
      readFromParent(&ID, sizeof(ID));
      for (unsigned i = 0, e = Breakpoints.size(); i != e; ++i)
        if (Breakpoints[i].first == ID) {
          Breakpoints.erase(Breakpoints.begin()+i);
          break;
        }
      break;
    }
    default:
      assert(0 && "Unknown command!");
    }
  }
}



/// startSubprogram - This method creates a new region for the subroutine
/// with the specified descriptor.
void Child::startSubprogram(void *SPDesc) {
  RegionStack = new SourceSubprogram(RegionStack, SPDesc);
}

/// startRegion - This method initiates the creation of an anonymous region.
///
void Child::startRegion() {
  RegionStack = new SourceRegion(RegionStack);
}

/// endRegion - This method terminates the last active region.
///
void Child::endRegion() {
  SourceRegion *R = RegionStack->getParent();

  // If the debugger wants us to stop when this frame is destroyed, do so.
  if (RegionStack == StopWhenSubprogramReturns) {
    StopAtNextOpportunity = true;
    StopWhenSubprogramReturns = 0;
  }

  delete RegionStack;
  RegionStack = R;
}




/// reachedLine - This method is automatically called by the program every time
/// it executes an llvm.dbg.stoppoint intrinsic.  If the debugger wants us to
/// stop here, we do so, otherwise we continue execution.  Note that the Data
/// pointer coming in is a pointer to the LLVM global variable that represents
/// the source file we are in.  We do not use the contents of the global
/// directly in the child, but we do use its address.
///
void Child::reachedLine(unsigned Line, unsigned Col, void *SourceDesc) {
  if (RegionStack)
    RegionStack->updateLocation(Line, Col, SourceDesc);

  // If we hit a breakpoint, stop the program.
  for (unsigned i = 0, e = Breakpoints.size(); i != e; ++i)
    if (Line       == Breakpoints[i].second.Line &&
        SourceDesc == (void*)Breakpoints[i].second.File &&
        Col        == Breakpoints[i].second.Col) {
      childStopped();
      return;
    }

  // If we are single stepping the program, make sure to stop it.
  if (StopAtNextOpportunity)
    childStopped();
}




//===----------------------------------------------------------------------===//
//                        Child class wrapper functions
//
// These functions are invoked directly by the program as it executes, in place
// of the debugging intrinsic functions that it contains.
//


/// llvm_debugger_stop - Every time the program reaches a new source line, it
/// will call back to this function.  If the debugger has a breakpoint or
/// otherwise wants us to stop on this line, we do so, and notify the debugger
/// over the pipe.
///
extern "C"
void *llvm_debugger_stop(void *Dummy, unsigned Line, unsigned Col,
                         void *SourceDescriptor) {
  TheChild->reachedLine(Line, Col, SourceDescriptor);
  return Dummy;
}


/// llvm_dbg_region_start - This function is invoked every time an anonymous
/// region of the source program is entered.
///
extern "C"
void *llvm_dbg_region_start(void *Dummy) {
  TheChild->startRegion();
  return Dummy;
}

/// llvm_dbg_subprogram - This function is invoked every time a source-language
/// subprogram has been entered.
///
extern "C"
void *llvm_dbg_subprogram(void *FuncDesc) {
  TheChild->startSubprogram(FuncDesc);
  return 0;
}

/// llvm_dbg_region_end - This function is invoked every time a source-language
/// region (started with llvm.dbg.region.start or llvm.dbg.func.start) is
/// terminated.
///
extern "C"
void llvm_dbg_region_end(void *Dummy) {
  TheChild->endRegion();
}




namespace {
  /// DebuggerIntrinsicLowering - This class implements a simple intrinsic
  /// lowering class that revectors debugging intrinsics to call actual
  /// functions (defined above), instead of being turned into noops.
  struct DebuggerIntrinsicLowering : public DefaultIntrinsicLowering {
    virtual void LowerIntrinsicCall(CallInst *CI) {
      Module *M = CI->getParent()->getParent()->getParent();
      switch (CI->getCalledFunction()->getIntrinsicID()) {
      case Intrinsic::dbg_stoppoint:
        // Turn call into a call to llvm_debugger_stop
        CI->setOperand(0, M->getOrInsertFunction("llvm_debugger_stop",
                                  CI->getCalledFunction()->getFunctionType()));
        break;
      case Intrinsic::dbg_region_start:
        // Turn call into a call to llvm_dbg_region_start
        CI->setOperand(0, M->getOrInsertFunction("llvm_dbg_region_start",
                                  CI->getCalledFunction()->getFunctionType()));
        break;

      case Intrinsic::dbg_region_end:
        // Turn call into a call to llvm_dbg_region_end
        CI->setOperand(0, M->getOrInsertFunction("llvm_dbg_region_end",
                                  CI->getCalledFunction()->getFunctionType()));
        break;
      case Intrinsic::dbg_func_start:
        // Turn call into a call to llvm_dbg_subprogram
        CI->setOperand(0, M->getOrInsertFunction("llvm_dbg_subprogram",
                                  CI->getCalledFunction()->getFunctionType()));
        break;
      default:
        DefaultIntrinsicLowering::LowerIntrinsicCall(CI);
        break;
      }
    }
  };
} // end anonymous namespace


static void runChild(Module *M, const std::vector<std::string> &Arguments,
                     const char * const *envp,
                     FDHandle ReadFD, FDHandle WriteFD) {

  // Create an execution engine that uses our custom intrinsic lowering object
  // to revector debugging intrinsic functions into actual functions defined
  // above.
  ExecutionEngine *EE =
    ExecutionEngine::create(new ExistingModuleProvider(M), false,
                            new DebuggerIntrinsicLowering());
  assert(EE && "Couldn't create an ExecutionEngine, not even an interpreter?");

  // Call the main function from M as if its signature were:
  //   int main (int argc, char **argv, const char **envp)
  // using the contents of Args to determine argc & argv, and the contents of
  // EnvVars to determine envp.
  //
  Function *Fn = M->getMainFunction();
  if (!Fn) exit(1);

  // Create the child class instance which will be used by the debugger
  // callbacks to keep track of the current state of the process.
  assert(TheChild == 0 && "A child process has already been created??");
  TheChild = new Child(M, EE, ReadFD, WriteFD);

  // Run main...
  int Result = EE->runFunctionAsMain(Fn, Arguments, envp);

  // If the program didn't explicitly call exit, call exit now, for the program.
  // This ensures that any atexit handlers get called correctly.
  Function *Exit = M->getOrInsertFunction("exit", Type::VoidTy, Type::IntTy,
                                          (Type *)0);

  std::vector<GenericValue> Args;
  GenericValue ResultGV;
  ResultGV.IntVal = Result;
  Args.push_back(ResultGV);
  EE->runFunction(Exit, Args);
  abort();
}